1. Field of the Invention
The invention relates to a chemical sensor device, a method for obtaining such chemical sensor device and a method for detecting analyte by using said chemical sensor device.
2. Description of Related Art
In recent years much effort has been made to develop devices, which mimic the sense of smell or taste. Such devices, which are usually called electronic noses and electronic tongues, respectively, would be well suited for a broad variety of applications, such as entertainment robots, identification systems, quality control systems, environmental monitoring, and medical diagnostics. However, up to now only a limited number of electronic nose devices have been marketed. Although these devices are capable of identifying or classifying some “odor” samples, further improvements are necessary to fulfill the needs for many advanced applications mentioned above. These applications often require higher sensitivity, higher discrimination capability, faster response, better stability, and lower power consumption. Since such features strongly depend on the characteristics of the chemical sensors used in the device, there is a strong demand for improved sensors meeting the requirements for advanced e-nose and e-tongue applications. An overview of sensor principles currently under development is given in J. W. Gardner and P. N. Bartlett, Electronic noses—Principles and applications, 1999, pages 67-116 Oxford University Press, Oxford.
There are several gas sensors available on the markets among which are metal oxide sensors, often referred to as Tagushi sensors. They are composed of metal oxide(s) having a porous form, generally doped with a metal. They are operated at elevated temperatures of 100 to 600° C. in order to allow combustion of the analyte at the metal oxide surface, inducing a change of oxygen concentration and therefore a change in conductance. Metal oxide sensors are generally employed as single device to detect toxic or flammable gases. They can also be employed as arrays for electronic noses, but their use for odor recognition was up-to-now limited by the lack of selectivity.
J. Kong, N. R. Franklin, C. Zhou, M. G. Chapline, S. Peng, K. Cho and K. Dai, Science, 2000, 287, 622-625 describe chemical sensors based on individual single-walled carbon nanotubes (SWNTs). Upon exposure to gaseous molecules such as NO2 or NH3, the electrical resistance of a semiconducting SWNT is found to change by up to three orders of magnitude within several seconds of exposure to analyte molecules at room temperature. The chemical sensors are obtained by controlled chemical vapour deposition growth of individual SWNTs from patterned catalyst islands on SiO2/Si substrates. Sensor reversibility is achieved by slow recovery under ambient conditions or by heating to high temperatures. After e.g. the NO2-flow is replaced by pure Ar, the conductance of the SWNT sample slowly recovers with a typical recovery time of about 12 hours at room temperature.
Z. W. Pan, Z. R. Dai and Z. L. Wang, Science, 2001, 291, 1947-1949, describe the synthesis of ultralong beltlike nanostructures, so-called nanobelts, of semiconducting oxides of zinc, tin, indium, cadmium, and gallium by evaporating the desired commercial metal oxide at high temperatures. The as-synthesized oxide nanobelts are pure, structurally uniform, and single crystalline, and most of them are free from defects and dislocations. They have rectangle cross section with typical width of 30 to 300 nanometers, width-to-thickness ratios of 5 to 10, and lengths of up to a few millimeters. A possible use of doped nanobelts as nanosize sensor is suggested.
V. Bondarenka, S. Grebinskij, S. Mickevicius, H. Tsardauskas, Z. Martunas, V. Volkov and. G. Zakharova, Phys. Stat. Sol., 1998, A 169, 289-294, have investigated the influence of humidity on the electrical properties of poly-vanadium acid xerogels and xerogels based on poly-vanadium acid where vanadium is partly substituted by molybdenum or titanium. The conductance of thin-film samples increases with an increase in humidity as an exponential function and therefore those films are suitable for the fabrication of humidity sensors. Thin films of the vanadium-metal-oxygen materials were produced by the sol-gel technology. The vanadium pentoxided powder and the other components were dissolved in hydrogen peroxide at 273K. Then the solution was heated in an open beaker at 353K for one/two hours. The obtained gels were deposited by a screen-printing method on substrates and baked at 333K in air. All compounds such obtained have a layered structure with interlayer distances of 11.1 to 11.5 angstroms. The amount of water contained in the compounds depends on the relative humidity RH and increases with an increase in RH.
S. Capone, R. Rella, P. Siciliano and L. Vasanelli, Thin Solid Films, 1999, 350, 264-268, investigated the physical and gas sensing properties of bulk material V2O5 and WO3 thin films. Gas-sensitive films of vanadium oxide and tungsten oxide were prepared by means of sputtering technique in a thickness of about 200 nm. Samples for gas testing were placed onto a heated sample holder and exposed to different gas concentrations. For both materials at high temperatures a strong exponential dependence of the electrical conductivity on the temperature was observed. Upon exposure to NO gas an increase of the electrical resistance of the films was observed. WO3-based sensors exhibited higher sensitivity values than V2O5 ones. In addition, tungsten oxide thin films were also able to detect very low concentrations of NO in the sub-ppm range. V2O5 could be used for detection of high concentration of NO, up to a range of 50-500 ppm.
Z. A. Ansari, R. N. Kareka and R. C. Aiyer, Thin Solid Films, 1997, 305, 330-335 describe a humidity sensor using planar optical waveguides with claddings of various oxide materials, among others bulk-V2O5. The planar waveguides were fabricated on a soda-lime glass substrate using an ion-exchange process. Films of porous semiconducting oxides were screen printed on the waveguide surface. The relative humidity (RH) was varied from 3 to 98%. At a cladding length of 3 mm and a cladding thickness of 25 μm V2O5 exhibited a response time of 5 secs. and a recovery time of 30 min. A hysteresis of 8% is observed for V2O5 cladding.
R. Rella, P. Siciliano, A. Cricenti, R. Generosi, L. Vanzetti, M. Anderle and C. Coluzza, Thin Solid Films, 1999, 349, 254-259, studied the physical properties and gas-surface interaction of bulk vanadium oxide thin films. Thin films of vanadium oxide were prepared by means of r.f. reactive sputtering. For evaluation of sensing properties the films were electrically tested in presence of different gases. Films grown with 15% oxygen in an Ar—O2-mixture exhibited best sensing properties, giving a maximum response at a working temperature ranging between 280 and 300° C.
In most cases vanadium pentoxide is only a secondary component in the sensitive coating employed in combination with a more sensitive material, e.g., WO3. X. Wang, N. Miura and N. Yamazone, Sensors and Actuators, 2000, B66, 74-76, report on WO3-based sensing materials for NH3 and NO detection. Gas sensing materials loaded with 1 wt.-% metal oxides were prepared. The sensing properties of these materials towards NH3 and NO were better than of sensing films of pure WO3.
The use of vanadium pentoxide films as temperature sensor is described by Z. S. El Mandouh and M. S. Selim, Thin Solid Films, 2000, 371, 259-263. The vanadium pentoxide films were prepared by an inorganic sol-gel method. The temperature coefficient of resistance, βT, is 2% K−1, which indicates, that V2O5 can be used as a thermoresistor.
WO98/26871 discloses nanotubes made from transitions metal oxides, preferably from a vanadium oxide of variable valence. The nanotubes show oxidation-reduction activities and are particularly suited as an active material for catalytic reactions. In the experimental part synthesis of vanadium oxide nanotubes and the structure of the nanotubes obtained is described.
WO01/44796 discloses a nanotube device comprising at least one nanotube, preferably a carbon nanotube, which is electrically connected with its ends to first and second conducting elements. The nanotube device may be used as a chemical or biological sensor. To tune the sensitivity of the device to a variety of molecular species the nanotubes may be modified by coating, or decorating with one or more sensing agents, so as to impart sensitivity to a particular species in its environment. The nanotubes may also be formed from other materials than carbon, e.g. silicon. Detection of various analytes is demonstrated in the experiments. Experiments were done on NO2 and NH3 gas, thioles, H2, CO and avidin (a protein). Modification of the sensitivity by depositing metal particles, e.g. gold, platinum of nickel, metal oxides, e.g. TiO2, or biological species on the sensing agent is also described.
Several types of sensors can be employed at room temperature and show good selectivity to organics. The most commonly encountered are conducting polymer chemiresistors polymer based SAW (Surface Acoustic Wave) and BAW (Bulk Acoustic Wave) devices. However, some of these sensors suffer from low sensitivity like for example conducting polymer chemiresistors to gases. Devices based on mechanical transducers like cantilever and BAW devices are harder to incorporate into integrated circuits than the ones based on electrical transducers. For optical detection based sensors, the complexity of the transducer may be a limiting factor, especially when miniaturization is considered. Concerning electrochemical cells, they are of limited use in the gas sensor domain but are gaining importance for electronic tongues.
A general problem in the use of sensors is humidity. Found in a large majority of samples it decreases the detection capabilities. The first reason is related to the fact that water will influence the analyte partitioning in the sensor medium or weaken the interactions of the analyte with the sensor medium. An example is the detection of an aroma of wine. One has to be capable of detecting traces of an aromatic compound among a matrix containing large amounts of water and alcohol. A second problem is that a change in humidity can be seen as a false detection. For example in the case of CO detection, a 20% change in relative humidity should not be interpreted as a 50 ppm CO.
A way to minimize the humidity problem is to dry the analyte. One can dehydrate the sample itself before analysis, for example dehydrating cheese before sensory analysis. The drawback is that the smell may denature during the process because volatiles are removed or decomposed. The headspace of the sample can also be dried before reaching the detector. This can for example be performed using a nafion filter. Water will be filtered off but some components of the analyte, like alcohols, will also be removed, partially or completely. Water can also be eliminated by separating the different chemicals of a sample using techniques like gas chromatography or similar techniques.
Only a limited number of reports exist where humidity is of advantage, meaning the sensors show an increase of sensitivity with increasing humidity. Kappler, J.; Tomescu, A.; Barsan, N.; Weimar, U.; Thin Solid Films 2001, 391, 186-191, report on an increase of sensitivity of SnO2 gas sensors operated at elevated temperature toward CO with increasing humidity. The sensor's response (Rair/Rco) increased from 5 to 30 by increasing the humidity from 0 to 50% relative humidity. Sadaoka, Y.; Sakai, Y.; Murata, Y. U.; Sensors and Actuators 1993, B 13-14, 420-423 report a similar behavior of an optical sensor based on calcein-poly(acrylonitrile) in the case of ammonia detection. The sensitivity increased when I/I0 (optical intensity ratio) decreased from 0.95 to 0.83 under dry air and 50% relative humidity, respectively. Another illustration is based on host molecules (tecton, DM 189) deposited on a mass-sensitive device (Boeker, P.; Horner, G.; Rosler, S. Sensors and Actuators 2000, B 70, 37-42). The response to 100 ppm ammonia (in Hertz) is double at 20.000 ppm water (saturated, humidity) compared to the response in dry air.
Amines are found in many foodstuffs, for example in wine, fish, cheese or meat. Amines can be for example indicators of fish freshness. Amines can also give some information on the health status of a person. There is therefore a need for amine sensors in the food industry and for medical applications. These sensors should be highly sensitive to the target preferably as well as show no significant decrease in sensitivity when humidity is present. An electronic nose comprising such sensors is therefore of great interest.
Some amine gas sensors are commercially available. For example electrochemical cells are offered on the market that are specific to a given amine, and that for a wide range of amines. The detection limit is around 2.5 to 5 ppm, depending on the amine. The main problem appears to be the size, which is in the centimeter scale. Metal oxide sensors can also detect ammonia, with a detection limit of about 25 ppm, but they suffer from their high power consumption and a low selectivity to amines.